1. Field of the Invention
The present invention relates to electronic musical instruments, and more particularly, to electronic musical instruments capable of generating varied musical tones like those of acoustic (non-electronic) musical instruments.
2. Prior Art
Due to recent technological improvements, tone generators employed in electronic musical instruments have come to be capable of synthesizing a wide variety of musical tones. For example, physical-modal tone-generators which synthesize sounds of acoustic musical instruments are conventionally known. Each of the physical-modal tone-generators simulates the sound production mechanism of a target acoustic musical instrument. Examples of the physical-modal tone-generators have been disclosed in Japanese Patent Application Laid-Open Publication No. 63-40199 and U.S. Pat. No. 4,984,276.
An example of conventional physical-modal tone-generators simulating the sound production mechanism of stringed instruments is shown in the block diagram of FIG. 19. In this figure, an excitation signal generating circuit 1 includes a waveform memory which stores excitation signal waveforms made up of a large number of frequency components, such as an impulse waveform. The excitation signal from the excitation signal generating circuit 1 is supplied to a first input terminal of an adder 2. The output signal from the adder 2 is supplied to a delay circuit 3 which simulates the delay of propagation of vibrating waves in a string of the target stringed instrument.
The delayed output signal from the delay circuit 3 is then supplied to a filter 4 which simulates acoustical losses of a vibrating string of the target stringed instrument. The output signal from the filter 4 is then supplied to a second input terminal of the adder 2. Elements 2 through 4 described above together form a closed loop. In addition to the delay circuit 3, the output signal from the adder 2 is supplied to a sound signal output terminal 5, whereby a signal circulating in the closed loop is output as a musical tone signal to be generated. With the conventional tone generator described above, when the excitation signal output from the excitation signal generating circuit 1 is supplied to the first input terminal of the adder 2, the supplied excitation signal begins to circulate in the closed loop. In this case, the signal circulates around the closed loop once in the time which is equal to the period of oscillation of the vibrating string being simulated, and the band width of the signal is limited by the filter 4 each time the signal passes through the filter 4. Then, the signal circulating in the closed loop is delivered as a musical tone signal from the musical tone output terminal 5. An example of the above-described type of tone generator has been disclosed in Japanese Patent Publication No. 58-48109.
In the above conventional electronic musical instrument comprising the above conventional tone generator, the variation of tone colors of the musical tones to be synthesized is limited and is a drawback in obtaining musical tone with a certain feeling of pitch and high quality, and in many cases where the pitch and the spectral construction of the excitation signal supplied to the closed loop must correspond to a pitch designated by a performer.
By increasing the loop gain of the closed loop (the filter with the comb-formed frequency characteristics), the comb-formed frequency characteristics of the entire closed loop for the signal circulating become sharper, so that the feeling of the pitch is improved. However, in this case, the stability of the operation of the closed loop decreases, and in the worst case, there is drawback in that the closed loop self-oscillates. Accordingly, there is a drawback in that the reliability of the system is degraded.